Booking a flight and rental car together offers the convenience of having your travel reservations stored on one website or with one company. Although you will likely receive separate confirmations from the airline and rental car company, you will be able to access your entire reservation in one place. Also, combining your bookings often means that travel companies can pass on preferential rates from third-party vendors and save you a substantial amount of money. If your travel preferences are flexible, check with several different airlines to get the best car-and-flight deal.
Rental cars booked with a flight are almost always drawn from the inventory at the car company's airport location. If you are traveling during peak times, you risk arriving at the desk to find that your chosen car has been rented. You may also be subject to airport taxes and fees in addition to your agreed-upon daily rate. Additionally, since most budget airlines and smaller car-rental companies don't list their rates on major travel sites or with travel agents, you may not be getting the best available deal.
If you book your flight and rental car well in advance, you'll have your pick of seats and cars in addition to access to low rates. You'll also enjoy the peace of mind that comes with knowing your reservations are complete. The closer your travel dates -- the later you wait to book -- the more reservations will have already been made with both your airline and car company of choice. Therefore, prices are likely to be higher for the remaining inventory.
Make sure that any savings you expect really are savings. For example, you may book a car rental with your flight at a substantially lower rate than a car by itself, but those savings may be erased if you find yourself responsible for extras like satellite radio, GPS services or additional insurance fees. Also, clarify the booking company's policy on changing your reservation. If you need to change your travel dates or reserve a larger car, for example, you may incur penalties.
